1. Field
The disclosure relates to a ceramic feedthrough for use in a high frequency signal transmission device. More particularly, a stripline transmission line is surrounded by a ceramic portion. This ceramic portion is then covered with a ground metallization.
2. Description of the Related Art
High frequency electronic packages are frequently hybrid packages containing a multiple devices. The hybrid package is solder attached to a circuit board for use in high data rate communication systems. A number of requirements are imposed on an interconnect that joins the hybrid package to the circuit board. First, the interconnect must be capable of transitioning the electrical signal from the location of integrated circuits inside the housing of the hybrid package to lower levels that make contact with the circuit board. The signal path from the integrated circuits to the circuit board must simultaneously provide good signal integrity, solderable connection to the circuit board and transverse the requisite distance.
Second, the hybrid packages require electrical interconnects with performance that is effective over a very wide frequency bandwidth, nominally from 0 to 50 GHz or even 80 GHz. This wide band performance is required to maintain signal integrity of the information carried along the electrical interconnects. If the signal integrity is not maintained, then the information carried along the electrical interconnects will become unusable and the information could be lost.
Third, the interconnects must be compatible with the solder used to attach to circuit boards using standard surface mount methods. Solder attach is important because it offers the board level system integrator flexibility in processes and a capability to use low cost manufacturing processes.
To demonstrate the importance of signal integrity, consider the effect of poorly performing interconnects. The effect may be illustrated by eye performance results from the signal passing through the electrical interconnect. In telecommunications, an eye diagram is an oscilloscope display that shows a digital data signal that is repetitively sampled. It is called an eye diagram because, for several types of coding, the pattern looks like a series of eyes between a pair of rails. It is an empirical method used for the evaluation of combined effects. It is commonly used for high speed communication signals as an indication of the quality of the signal. For interconnects, the eye diagram shows undesired effects within the interconnects that will degrade the signal performance.
Several signal performance measures can be derived by analyzing the eye diagram. If the signals are too long, too short, poorly synchronized with the system clock, too high, too low, too noisy, too slow to change, or have too much undershoot or overshoot, this can be observed from an eye diagram. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to intersymbol interference and noise appears as closure of the eye pattern. An exemplary eye diagram with good performance is illustrated in FIG. 1. The eye 2 has symmetry and is uniform in shape.
The eye diagram illustrated in FIG. 2 shows the effect of transmission line dispersion, an electrical interconnect effect that degrades eye performance. Dispersion causes the transmission line propagation constant to be non-linear with frequency and line impedance to change as a function of frequency. The eye 4 lacks symmetry and is not uniform in shape. Other effects that degrade eye diagram performance include a stray inductive or capacitive parasitic as part of the electrical interconnect.
One prior high speed interconnect is a metal box with high quality coaxial connectors. The connectors are approximately 0.5 inch×0.5 inch×0.25 inch and require coaxial cable for connection. While these connections are capable of providing high quality interconnect performance over a very broad frequency range, they are not surface mountable. The connectors are too large to effectively integrate with a compact telecommunications equipment box.
A high speed interconnect is disclosed in U.S. Pat. No. 8,933,450, titled “High-Frequency Transmitting Device,” by Okumichi et al., that is incorporated by reference herein in its entirety. FIGS. 1A-1D of U.S. Pat. No. 8,933,450 illustrate a microstrip or coplanar waveguide transmission line forming a vertical transition from a top layer to a bottom layer. This approach utilizes a coaxial section that allows transition of the electrical signal from a layer with integrated circuits down to a layer that contacts with a circuit board. The coaxial section is formed using a center conductor via and a series of vias that form an electrical ground contact. This permits the signal to travel over transmission lines that are matched or nearly matched to the required system impedance, that is normally 50 ohms. A drawback with this approach is that it requires the electrical system to make an abrupt 90° bend in at least two locations which degrades electrical performance. The 90° bends occur at transition points at the top and at the bottom of the ceramic stack. One attempt at addressing this drawback is to use a stair-stepped via structure as mention in U.S. Pat. No. 8,933,450. This approach adds additional transition discontinuities which must be compensated and degrade electrical performance. Another drawback is that it requires a transition from co-planar waveguide (CPW), or microstrip, transmission line to coaxial transmission at two locations. This transition further degrades the electrical performance because the electric field distributions are so different between horizontal CPW or microstrip and the vertical coaxial line.